If some day I want to remove the replay gain from all my files, how should I do ?
Select all the songs in your library, right-click on one of the selected songs and select "Tag" from the menu, under the Tag pop-up select "Show All Tags", find the replay gain tag, select what's inside the box (should be "[Varies]"), and delete it. That will delete the tag from the library. If you want to delete the tag from all the files as well, you need one additional step: right-click on one of the selected songs again, go to and select Library Tools/Update Tags (from library).
(Note: I originally said "Except it doesn't seem to work with MC15 - when I select all the songs and go to the Tag box, it only ever shows the standard 8 or so tags, even when I select "Show All Tags". Works fine with MC14." In fact it does work fine with MC15 - there was something wrong with my installation that was fixed by a complete uninstall/re-install of MC15.)
-Fred